This invention relates to information recording media in the form of light-readable discs, and more particularly to such media which are made up of two such discs secured to one another back to back to form a two-sided stack.
Light-readable information recording media in the form of discs are well known as shown, for example, by Kramer U.S. Pat. No. 5,068,846. Commercially available compact discs ("CDs") and compact disc read-only memories ("CD-ROMs") are examples of recording media of this general type.
More recently, digital versatile discs ("DVDs") have emerged wherein light-readable information technology is employed to record movies and other similar real-time audio/visual programming (hereinafter "program information") on discs. To increase program information storage capacity, DVDs are formed from two discs (similar in appearance to conventional audio CDs) bonded back to back (see, for example, U.S. Pat. No. 5,729,533, issued Mar. 17, 1998, the entire contents of which are hereby incorporated by reference). Bonding of the two discs is normally performed by coating each disc's back side (i.e., the side of a disc which is bonded to another disc) with a light-curable bonding material, stacking the discs back to back, and curing the bonding material by simultaneously exposing each disc's front side to ultra-violet light. This bonding technique is commonly referred to as "simultaneous exposure."
When the two discs forming a DVD are structurally symmetrical or otherwise structurally similar (e.g., both discs contain program information), the transmissivity of each disc is also similar. Accordingly, light of approximately the same characteristics (e.g., wavelength and intensity) may be used to expose each disc during simultaneous exposure. However, when the discs to be bonded are structurally asymmetrical or otherwise structurally dissimilar (e.g., one disc contains program information and the other disc contains printed images or other artwork), the transmissivity of each disc may differ greatly. As a consequence, during the simultaneous exposure of a DVD formed with structurally asymmetrical or structurally dissimilar discs (hereinafter "an asymmetrical DVD"), unequal amounts of light reach the bonding material through each disc (assuming a similar light source is used to expose each disc). For example, when a disc contains a printed image, the characteristics of light transmitted through the disc vary with the transmissivity variations inherent in the printed image (e.g., due to ink thickness or color variations). By curing through such an image, the underlying bonding material will be cured non-uniformly and in a manner related to the spatial transmissivity variations present in the printed image. In such a case, an image closely resembling the original printed image (e.g., a mirror image of the original image) may be visible through a program information disc bonded to the printed image disc. This type of bleed-through phenomenon is referred to as "image pass-through." In addition to producing an unattractive mirror image of the original printed image, the non-uniformly cured bonding material responsible for image pass-through may actually degrade DVD playability.
One technique for reducing image pass-through in an asymmetrical DVD is to cure the light-curable bonding material by exposing only the program information side of the DVD to light (commonly referred to as "one-sided exposure"). In this manner, the spatial transmissivity variations present in the printed image have no effect on the curing process, and image pass-through is eliminated. Unfortunately, a one-sided exposure also results in non-uniform curing of the bonding material as bonding material proximate the disc being exposed to light is cured more rapidly than bonding material distal the exposed disc. Because bonding material expands during the curing process, bonding material receiving more light (e.g., the bonding material proximate the exposed disc) expands more rapidly than bonding material receiving less light (e.g., the remaining bonding material). This non-uniform bonding material expansion strains the bonding material and the discs of the DVD and causes the DVD to warp or tilt. Thereafter, the warped/tilted DVD will not play properly. This problem is exacerbated by the non-uniform infrared heating associated with the curing process.
A need therefore exists for methods for bonding structurally asymmetrical or otherwise structurally dissimilar optical discs which minimize both disc tilt and image pass-through. Such methods will allow a program information disc to be bonded to a graphical image disc without experiencing the drawbacks associated with either phenomenon.
In view of the foregoing, it is an object of this invention to provide methods for bonding structurally asymmetrical or otherwise structurally dissimilar optical discs which reduce both disc tilt and image pass-through.